I visited Santa Ana and San Ignacio Mini on my way from Posadas to Iguazu Falls.  Posadas, a city across the river from Paraguay, is a fine but unspectacular city, and I used it as a base to explore the worthwhile Parque Nacional Ibera, where you will see oodles of capybaras.

San Ignacio Mini, which sits in middle of the small town, boasts beautiful carvings.  Although San Ignacio Mini is the most popular of the three Argentinian Jesuit missions among the Guaraní sites, I did not find it to be very crowded (July 2019).  The visit itself can be done in an hour, unless  you come across - as I did - a rolling swarm of caterpillars for the first time in your life and squat in middle of the ruins to observe it for longer than most people would.

Santa Ana (photo) is less grandiose and lacks the "wow" factor of San Ignacio Mini, but it is more quietly atmospheric.  Very little of the Santa Ana structures have been restored, but they have been stabilized.  Beside the ruins of the church is a more recent but neglected cemetery, which adds to the haunting feel of the ruins.

In summary, I found the two ruins to be very deserving stops (I spent a night in San Ignacio) on the way to Iguazu Falls.  Although I enjoyed Santa Ana very much, if you only had time to visit one of them, I would prioritize San Ignacio Mini because it is a more complete and extensive site with a rolling swarm of caterpillars.